1. Field of the Invention
The present invention relates to a method of producing a base plate for a circuit board. In particular, the present invention relates to a method of producing a base plate which has an insulating layer formed on its surface and is used for a circuit board of a semiconductor device or the like.
The present invention also relates to a base plate for a circuit board and a circuit board using the base plate.
2. Description of the Related Art
Known examples of a method of forming a circuit board involving the formation of an insulating layer on the surface of a base plate include a method as shown in JP 06-310825 A involving the steps of applying an insulating material made of a resin or the like to the surface of a base plate and curing the applied insulating material. A circuit board formed as described above is used as a semiconductor device by joining through a solder a semiconductor element on a wiring layer formed on the surface of the insulating layer.
A base plate formed of Al or the like having a high coefficient of thermal conductivity has been generally used in a semiconductor device for the purpose of efficiently dissipating heat generated in a semiconductor element to the outside. However, there is a large difference in coefficient of thermal expansion between a semiconductor material such as Si used for the semiconductor element and Al forming the base plate. As a result, a thermal stress is generated between the base plate and the semiconductor element upon a change in temperature. Therefore, the semiconductor element may warp, or a crack may develop in solder for joining the semiconductor element.
In view of the above, the use of a base plate formed of an Al/SiC composite having an excellent coefficient of thermal conductivity but having a small coefficient of thermal expansion has been recently proposed to alleviate a thermal stress in a semiconductor device.
A base plate formed of such Al/SiC composite is produced by casting, but casting blowholes are known to occur on the surface of the base plate or in the base plate at the time of casting. Therefore, when an insulating layer is formed on the surface of the base plate by means of the above-described method such as application, the insulating layer is affected by the casting blowholes on the surface of the base plate. As a result, the insulating layer includes some portions having a thickness smaller than a predetermined thickness, so desired insulating property may not be secured.
In contrast, the desired insulating property can be secured by sealing casting blowholes on the surface of a base plate by means of surface grinding, impregnation with a resin, or the like and thereafter forming an insulating layer on the surface of the base plate. In the case of surface grinding, however, the cost for grinding results in an increase in total cost, and the size of the base plate cannot be increased by reason of equipment. In the case of impregnation with a resin, the casting blowholes is sealed with a resin having a lower coefficient of thermal conductivity and a larger coefficient of thermal expansion than those of a metal. As a result, the base plate will have a reduced coefficient of thermal conductivity and an increased coefficient of thermal expansion, thereby reducing thermal property values of the entire base plate.
Also, there is a method of joining a heat sink with a base plate to efficiently dissipate generated heat. The base plate and the heat sink may be joined with each other by means of silicone grease. However, the coefficient of thermal conductivity of the silicone grease is generally as low as 0.8 W/mK, thereby resulting in an increase in thermal resistance due to the grease.